Offsets between H-alpha and CO arms of a spiral galaxy NGC 4254: A New Method for Determining the Pattern Speed of Spiral Galaxies
Abstract
We examined offsets between HII regions and molecular clouds belonging to spiral arms of a late type spiral galaxy NGC 4254 (M99). We used a high resolution CO(1-0) image obtained by Nobeyama Millimeter Array (NMA) and an H-alpha image. We derived angular offsets (theta) in the galactic disk, and found that these offsets show a linear dependence on the angular rotation velocity of gas (OmegaG). This linear relation can be expressed by an equation: theta =(OmegaG - OmegaP) * tH-alpha, where OmegaP and tH-alpha are constant. Here, OmegaP corresponds to the pattern speed of spiral arms and tH-alpha is interpreted as the timescale between the peak compression of the molecular gas in spiral arms and the peak of massive star formation. We may thus determine OmegaP and tH-alpha simultaneously by fitting a line to our theta - OmegaG plot, if we assumed they are constant. From our plot, we obtained tH-alpha =4.8 (+/- 1.2) Myr and OmegaP = 26 (+10/-6) km/s/kpc, which are consistent with previous studies. We suggest that this theta - OmegaG plot can be a new tool to determine the pattern speed and the typical timescale needed for star formations.
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